Coordinate regulation of cell shape changes and intercellular adhesion is critical for morphogenesis, cell migration, survival and growth during development. Uncoupling of these processes results in developmental defects that lead to devastating pathological conditions, including cardiovascular abnormalities. The goal of this research is to define the mechanisms employed by Abi protein complexes and associated Abl kinases to regulate actin dynamics downstream of adhesion and growth factor receptors during developmental morphogenesis. We showed that Abl kinases play a role in the formation and turnover of actin structures downstream of cell surface receptors. These processes are driven by localized actin polymerization, which we have demonstrated involves the activity of the Abi proteins. We have shown that the Abi/Nap1 complex is required for the formation and stability of cell-cell contacts, and that Abi exists in distinct protein complexes: one, which is comprised of two Abl-binding proteins, Abi and Wave, as well as Nap1, Sra1 and Arp2/3, and a second complex comprised of distinct novel Abi-binding proteins, which is required for cell-cell adhesion. Notably, mice deficient for Abl family kinases, Wave2, Abi1, and Nap1 exhibit striking developmental defects consistent with altered cytoskeletal dynamics and defective intercellular adhesion, leading to cardiovascular abnormalities. A fundamental gap in knowledge exists regarding the signaling pathways linking intercellular adhesion to actin polymerization. The specific aims of this proposal are: 1) to elucidate the mechanisms employed by Abi complexes to regulate morphogenesis downstream of adhesion receptors using in vitro models and knockout mice, with emphasis on our recently discovered Abi/Nap1-dependent and Arp2/3- independent pathway, which is critical for the integrity of cell-cell junctions;and 2) to define the mechanisms whereby the Abl kinases regulate cell proliferation, survival and adhesion in endothelial cells, with emphasis on trafficking of adhesion receptors. Significance: Results from the proposed studies will provide novel insights into the mechanisms whereby components of the actin polymerization machinery regulate adhesion processes critical for endothelial cell growth and survival. Moreover, our findings will provide much needed knowledge for elucidating the molecular basis of develomental abnormalities such as congenital cardiovascular disease and other pathologies induced by disregulated actin dynamics.